1. Field of the Invention
The invention relates to image processing systems. More particularly, the invention relates to a method for accurately transforming color information between two color spaces having differing dimensions, e.g., between a red-green-blue (RGB) color space and a cyan-magenta-yellow-black (CMYK) color space and vice versa.
2. Description of the Background Art
In printing, image retouching and image processing, it is often necessary to convert colors from one representation (color space) into another. Many computer video monitors and scanners, for example, use red-green-blue (RGB) representations for colors, while printers typically represent colors in terms of the amounts of a variety of differently colored inks (for example, cyan-magenta-yellow-black (CMYK)). As such, in a typical computer system, the RGB color space used to produce an image upon a computer screen must be converted into a CMYK color space to facilitate printing of the image depicted on the screen. However, for any particular two color spaces, it is in many instances much easier to convert in one direction than the other, e.g., convert from CMYK to RGB. For example, converting from CMYK to RGB is relatively easy because the CMYK space has more dimensions than the RGB space.
Specifically, an important task in photocompositing is to take a set of images in CMYK format, modify them, and output the result in CMYK. Many of the intermediate operations (image modifications) are more easily or effectively accomplished in RGB space, so it is often necessary to convert from CMYK to RGB and then back to CMYK. One problem with such a transformation is that a CMYK color space is a four-dimensional space and an RGB color space is a three-dimensional space, so the transformation from CMYK to RGB, though relatively simple, inherently loses color information. In particular, the color information produced by "black generation" during creation of the CMYK image is lost. Black generation describes an amount of black ink substituted for equal parts of Cyan, Magenta and Yellow for printing purposes. Consequently, it is very important that an image processing system be able to convert from CMYK to RGB and back to CMYK and produce a black component of the CMYK image that closely approximates the black component in the original CMYK image.
Thus, a difficult and widely needed color transformation is the transformation from an RGB color space to a CMYK color space that retains, as closely as possible, the black generation of the original CMYK image. U.S. Pat. No. 4,500,919 discloses a particular method for converting from RGB to CMYK which is called the Ink Correction Model (ICM). The patent mentions that the ICM ". . . could be implemented in one huge lookup table, but this would be uneconomic and would also give no clue as to how to find the data to be stored in the LUT short of an impossibly large printing test." [11:21] Since the time of filing of '919 patent, the cost of memory has been reduced sufficiently that it is no longer uneconomic to use "one huge lookup table". Furthermore, the '919 patent states that, in using a table based transformation, a large printing test must be conducted to facilitate color space transformation calibration. However, such printing tests are time consuming and complicate the transformation process.
Therefore, a need exists in the art for a method that rapidly and accurately transforms images from a first multi-dimensional color space, e.g., RGB, into a second multi-dimensional color space, e.g., CMYK, without using a printing test and which preserves as closely as possible the black generation of an original CMYK image.